An RFID system enables radio communications to be carried out between an RFID tag equipped with an IC chip and an RFID reader/writer. RFID tags include an active-type tag in which a battery is mounted and runs from the electric power of the battery, and a passive-type tag that receives electric power from a reader/writer and runs from this electric power. Because an active-type tag is equipped with a battery, it has merits in view of a communication range, communicative stability, etc., compared with a passive-type tag, while because an active-type tag has a complicated structure, it has demerits of being upsized, increasing in cost, and so on. Recent improvements in semiconductor technologies advance the downsizing of IC chips used for passive-type tags and the enhancement of their performance, and there occurs a situation in which the use of passive-type tags in a broad field is expected from extension of their communication ranges, improvements in the communicative stability, etc.
In an electromagnetic-induction-type RFID tag included in passive-type RFID tags and having a frequency band which is a low or high one, a voltage is induced in the RFID tag through an electromagnetic induction action between the transmission antenna coil of a reader/writer and the antenna coil of the RFID tag and the IC chip of this tag starts from this voltage, so that communications can be carried out. Therefore, the RFID tag operates only within the induction field caused by the RFID reader/writer, and its communication range is of the order of about tens of centimeters. Furthermore, in an RFID tag having a high frequency band, such as a UHF band or a microwave band, because a radio frequency communication method is applied and electric power is furnished to the IC chip of the RFID tag by using an electric wave, its communication range is greatly improved by up to about 1 to 8 meters. Therefore, an RFID system using RFID tags having a high frequency band, such as a UHF band or a microwave band, can carry out batch reading of plural RFID tags, reading of an RFID tag which is moving, etc., which are difficult for an RFID system having a low frequency band or a high frequency band and having a short communication range to implement, and it is considered that the use area of such an RFID system using RFID tags having a high frequency band, such as a UHF band or a microwave band, will widen greatly from now on. As passive-type tags having a high frequency band, such as a UHF band or a microwave band, there have been provided some tags described in, for example, patent references 1 to 6.
In FIG. 3 of patent reference 1, there is disclosed a conventional RFID tag including a ½-wave microstrip line resonator 13, a dielectric substrate 14, and a ground conductive plate 15, which can be mounted and secured onto a metal body (a conductor) with the radiation characteristic of the antenna being hardly affected even if the metal body (the conductor) is placed on a side of the ground conductive plate 15 because an IC chip is connected between the ½-wave microstrip line resonator 13 and the ground conductive plate 15. In FIG. 17 of patent reference 1, there is disclosed the details of the IC chip in which an IC 106 is placed in such a way as to be buried in a dielectric material on the rear surface of a ground conductive line 104 connected to a central section of an antenna conductor 100 on a side of a ground conductor by using a technology such as wire bonding. Similarly, in FIGS. 18 to 21 of patent reference 1, there is disclosed a tag in which an IC 121 is placed in such a way as to be buried into a dielectric material on a side of a ground conductor. In paragraph number 0028 of patent reference 1, there is a description showing that, when the IC 121 has an IC structure in which a ground surface can be formed opposite to a surface on which a connection pad is formed, one bonding wire 122, among bonding wires 122, 12, and 2, can be eliminated.
In FIG. 1 of patent reference 2, there is disclosed an RFID tag provided with a terminal portion 3 formed on a front surface of a substrate 1, and an IC chip 6 that is placed in an IC chip placement area 9 formed in a part of the substrate 1 and is connected to the terminal portion 3. The patent reference then discloses that because it is not necessary to connect the IC chip 6 with wire bonding, it is not necessary to embed the IC chip 6 into the inside of the substrate 1, and therefore the IC chip 6 can be mounted on an antenna upper surface, the RFID tag having the simple structure can be manufactured through only machining of the front surface of the substrate 1, and reduction of the yield and reduction of the manufacturing cost can be achieved.
In FIG. 19 of patent reference 3, there is disclosed an RFID tag 5 equipped with a dielectric member 10, an recessed portion 10b for IC chip, a film base 20, an antenna pattern 30, and an IC chip 40, in which the recessed portion 10b for IC chip in which the IC chip 40 can be buried is formed in the dielectric member 10 and the IC chip 40 is buried in this recessed portion 10b for IC chip, and the film base 20 is wound around the dielectric member 10 in such a way that the antenna pattern 30 formed on an inner surface of the film base 20 is electrically connected to the IC chip 40, so that reduction of its communication range even in a vicinity of a wave absorber can be suppressed by a loop antenna constructed of the antenna pattern 30.
In FIGS. 1 and 2 of patent reference 4, there is disclosed an RFID tag in which an IC chip 21 is fitted into a recessed portion 15 of a base material 11 (a substrate), and the both ends of an antenna pattern 13 formed with screen stencil using a conductive ink are electrically connected to the IC chip 21. Furthermore, in FIG. 4 of patent reference 4, there is disclosed an RFID tag having a pair of two antenna patterns 13A and 13B.
In FIG. 4 of patent reference 5, there is disclosed an RFID tag in which an opening 31 through which a part of a dielectric substance 20 is exposed is formed in an antenna surface 30, the opening has a pair of first slits 31a extending opposite to each other in parallel, the pair of slits 31a, and a second slit 31b connecting the pair of slits 31a, and the above-mentioned second slit 31b is positioned in a midpoint portion of the above-mentioned pair of first slits 31a. In a transmitting and receiving element (an IC chip), first and second feeding points are connected to 41 and 42 respectively.
In FIGS. 1 and 2 of patent reference 6, there is disclosed an RFID tag in which an IC chip 13 is mounted in a slot antenna 10 which is constructed in such a way that a slot 12 is disposed in a central portion of a rectangular conductive plate 11 with respect to a direction of the length of the rectangular conductive plate.    [Patent reference 1] JP,2000-332523,A (FIGS. 3 and 17 to 21)    [Patent reference 2] JP,2002-197434,A (FIG. 1)    [Patent reference 3] JP,2006-53833,A (FIG. 19)    [Patent reference 4] JP,2003-223626,A (FIGS. 1, 2 and 4)    [Patent reference 5] JP,2006-237674,A (FIG. 4)    [Patent reference 6] JP,2002-358494,A (FIGS. 1 and 2)
The RFID tag disclosed by patent reference 1 can be mounted onto a metal body (a conductor). However, because the RFID tag is constructed in such a way that the IC chip is connected between the ½-wave microstrip line resonator and the ground conductive plate, it is necessary to connect the IC chip with wire bonding and to embed the IC chip into the inside of the dielectric substrate. Therefore, although there is a low possibility that the IC chip is damaged by a shock or the like, a problem is that the structure of the RFID tag is complicated and the manufacture (mass production) of the RFID tag becomes difficult.
In the RFID tag disclosed by patent reference 2, the thickness of the IC chip is large compared with the thickness of the antenna pattern and that of the conductor of the terminal portion even if the downsizing of the IC chip proceeds, and a projection appears on the front surface of the RFID tag because the IC chip is mounted on the surface of the base material. Therefore, as described in paragraph number 0023 of patent reference 2, it is necessary to cover and protect the whole or a part of the mounted portion of the IC chip to make the front surface of the RFID tag flat. That is, a problem is that when mounting the antenna pattern and the IC chip on the base material, there is a possibility that the IC chip is damaged by a shock or the like, and it is therefore difficult to print a label directly on the front surface (the upper surface) of the RFID tag by using a label printer. Furthermore, even in a case in which a film on which the antenna pattern and the IC chip are mounted is bonded onto the base material, the problem as mentioned above arises because a bulge (a projection) occurs in the film due to the mounting of the IC chip.
A problem with the RFID tag disclosed by patent reference 3 is that although a bulge (a projection) hardly occurs in the film (the film base) due to the mounting of the IC chip, in a case in which the RFID tag is attached to a conductive object (a conductor) such as a metal body or is mounted in a vicinity of a conductive object, the loop antenna stops operating under the influence of the conductive object, and the communication range drops extremely.
A problem with the RFID tag disclosed by patent reference 4 is that although a bulge (a projection) hardly occurs in the film due to the mounting of the IC chip, in a case in which the RFID tag is attached to a conductive object (a conductor) such as a metal body or is mounted in a vicinity of a conductive object, the loop antenna or dipole antenna stops operating under the influence of the conductive object, and the communication range drops extremely, as in the case of RFID tag disclosed by patent reference 3.
The RFID tag disclosed by patent reference 5 can be mounted onto a metal body (a conductor), like the RFID tag disclosed by patent reference 1. However, because the IC chip is disposed outside the dielectric substance 20, the thickness of the IC chip is large compared with the thickness of the antenna pattern and that of the conductor of the terminal portion even if the downsizing of the IC chip proceeds, and a projection appears on the front surface of the RFID tag because the IC chip is mounted on the surface of the base material. For this reason, as in the case of the RFID tag disclosed by patent reference 2, a problem is that there is a possibility that the IC chip is damaged by a shock or the like, and it is therefore difficult to print a label directly on the front surface (the upper surface) of the RFID tag by using a label printer. Furthermore, in a case in which a film on which the antenna pattern and the IC chip are mounted is bonded onto the base material, the problem as mentioned above arises because a bulge (a projection) occurs in the film due to the mounting of the IC chip. In addition, because the opening has the pair of first slits 31a extending opposite to each other in parallel, the pair of slits 31a, and the second slit 31b connecting the pair of slits 31a, and this opening 31 is constructed in such a way that regions 36 and 37 on the antenna surface 30 which are limited by the dielectric substance 20 exposed via the opening 31 forms a matching circuit used for the transmitting and receiving element, the pair of slits 31a has a horizontally-oriented shape with respect to the lateral direction which is the feed direction and an electric field of cross polarization in the longitudinal direction also occurs in the pair of slits 31a in addition to an electric field of correct polarization in the lateral direction occurring in the second slit 31b, and therefore the gain of the correct polarization component is reduced.
A further problem is that because the cross polarization component occurring is radiated in a direction different from the direction which is intended originally for the correct polarization, there is a case in which the tag communicates with the reader/writer even though the tag is placed at a location where the reader/writer does not desire to communicate with the tag, so that the implementation of the mounting method of mounting the tag and the managing method of managing the tag becomes difficult. In addition, a problem with the patch antenna of the RFID tag disclosed by patent reference 5 is that although the feeding points 41 and 42 are located in a vicinity of the center of the antenna face 30, the pattern of the correct polarization wave also becomes asymmetrical and therefore affects the symmetry of the radiation pattern of the antenna because the patch antenna is based on the placement of the slit at a position shifted from the center of the antenna face 30. It can be seen from these problems that the patch antenna disclosed by patent reference 5 is focused on the establishment of the matching between the regions 36 and 37 and the transmitting and receiving element (the IC chip).
A problem with the RFID tag described in FIGS. 1 and 2 of patent reference 6 in which the slot antenna having a slot formed within the conductive pattern is applied is that in a case in which the RFID tag is attached to a conductive object (a conductor) such as a metal body or is mounted in a vicinity of a conductive object, the loop antenna or dipole antenna stops operating under the influence of the conductive object, and the communication range drops extremely, as in the case of RFID tag disclosed by patent reference 3. In addition, a problem is that because a magnetic field occurs in a direction of the length of the slot shown in FIG. 1 of patent reference 5, and the RFID tag is resonated and radiates an electric wave with this length, the slot has to have a length of the order of about λ/2 in order that the RFID tag radiates an electric wave with a high degree of efficiency, and the downsizing of the RFID tag becomes difficult.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an RFID tag that can be mounted to an object regardless of whether the object is a conductive one or a non-conductive one without shortening its communication range, and a new RFID tag that, in addition to the above-mentioned capability, can reduce the possibility of an IC chip being damaged due to a shock or the like because the RFID tag does not have any projection on any surface thereof due to the mounting of the IC chip thereto, and that is easy to be subjected to post-machining (adjustment) after the RFID tag is manufactured.
It is another object of the present invention to provide a new RFID tag that can be mounted to a flat or curved mounting surface of an object regardless of whether the object is a conductive one or a non-conductive one without shortening its communication range, that can reduce the possibility of an IC chip being damaged due to a shock or the like because the RFID tag does not have any projection on any surface thereof due to the mounting of the IC chip thereto.